1. Field of the Invention
The present invention relates to a control apparatus for controlling a vertical vibration elimination table which is used as a component in a semiconductor production unit and which is of the type having four supporting leg structures. More particularly, the present invention relates to a vertical vibration eliminator control apparatus which effectively performs control of motion of four degrees of freedom: three degrees of freedom in a rigid motion mode including one degree of freedom for translational motion and two for rotational motion, plus one degree of freedom for flexible motion.
2. Description of the Related Art:
In general, a pneumatic vibration elimination table carries apparatuses or instruments which are to be kept isolated from vibration. An example of such an apparatus is an exposure XY-stage which is used in the production of semiconductors. In order to achieve precise and quick exposure, the X-Y stage has to be used on a vibration elimination table which is free as much as possible from vibration. This is because the exposure has to be conducted while the exposure X-Y stage is completely still. The operation of the X-Y stage essentially includes intermittent motion of step and repeat. Thus, the X-Y stage itself constitutes a source of vibration as it generates vibration during a repeated step operation to vibrate the vibration elimination table. Exposure operation cannot be commenced until the vibration caused by the X-Y stage remains in the vibration elimination table. Thus, the vibration elimination table is required to have such characteristics that provide good balance between isolation from external vibration and damping of vibration forcibly caused on the table by, for example, an X-Y stage.
As well known in the field, a control apparatus for a pneumatic vibration elimination table employs an active feedback control in which the table is driven in such a manner as to cancel detected displacement of the vibration elimination table caused by vibration. FIG. 2 is a block diagram of a conventional control apparatus for a pneumatic vertical vibration elimination table of a type in which vertical position control of a vibration elimination table 8 is effected by means of four pneumatic leg structures. The pneumatic supporting leg structures are arranged at four corners of the vibration elimination table 8. Referring to FIG. 2, there are shown a servo valve 1a which controls supply discharge of air as the working medium to and from a pneumatic spring 2a, a position sensor 3a for measuring vertical displacement of a pedestal 4a, an acceleration sensor 5a for measuring vertical acceleration of the pedestal 4a, a pre-loading mechanical spring 6a and a viscous element 1a which inclusively represents the viscosity of the entire mechanism including the pneumatic spring, mechanical spring and other elements which are not shown. The mechanism constituted by the components or elements 1a to 7a is collectively referred to as a pneumatic spring supporting structure. As shown in this Figure, four pneumatic spring supporting leg structures are used to vertically support the vibration elimination table 8. Suffix symbols "a", "b", "c" and "d" are attached to numerals indicating the elements or components for the purpose of discrimination.
A description will now be given of the construction and operation of a feedback apparatus 15a for the pneumatic spring supporting leg structure having the components 1a to 7a. Output from the acceleration sensor 5a is negative feedback, fed through a low-pass filter 9a having a moderate amplitude and time constant, to the primary side of a voltage-current converter 10a which is used for operating the servo valve 1a. This acceleration feedback loop serves to stabilize the operation of the entire mechanism and produces a damping effect. The output from the position sensor 3a is input to a comparator circuit 12a trough a displacement amplifier 11a so as to be compared with a command voltage 13a which is equivalent to a command position of the pedestal 4a relative to the ground level, and the difference is output as a position error signal e.sub.a. This position error signal e.sub.a is supplied through a PI compensator 14a to excite a voltage current converter 10a. Consequently, the servo valve 1a is opened and closed in accordance with the position error signal, so that the pressure inside the pneumatic spring 2a is correspondingly adjusted so as to hold the pedestal at the command position with zero steady-state error P and I respectively indicate proportional and integrating operations. The other three pneumatic spring supporting leg structures are controlled by feedback systems 15b, 15c and 15d similar to the feedback system 15a described above.
As stated before, the vibration elimination table carries instruments or apparatuses which are to be kept as much as possible from vibration. The vibration elimination table 8 therefore must be constructed to have high rigidity. Such a rigid construction causes the weight of the whole system including the vibration elimination table 8 and instruments and apparatuses carried by the table to be increased tremendously. In addition, transportation and installation of the vibration elimination table is dangerous and requires great care because the table including the associated control apparatuses is large and heavy and because the table is designed and constructed delicately to carry various types of precision equipment. Consequently, a specific transportation facility is required for the purpose of transportation of the vibration elimination table. It is also necessary that the construction of the site where the vibration elimination table and associated apparatuses are to be situated has a high level of rigidity and strength to bear the large weight. More specifically, a house having a floor with a large load carrying capacity is required for accommodating the vibration elimination table.
Thus, the rigid construction of the vibration elimination table 8 not only raises the cost of the table 8 itself but also incurs increased overall costs inclusive of the costs for transportation, installation and building of the accommodation facility such as a house.
Japanese Unexamined Patent Publication No. 3-28910 discloses a control method for a pneumatic vibration elimination table which uses a state feedback control. The state feedback method, however, too strongly relies upon physical parameters of the controlled object, so that this technique cannot suitably be used in practical commercial or industrial systems.